1. Field of the Invention
This invention relates to printed wiring board substrates having extremely low thermal expansion coefficients. The printed wiring board substrates are particularly useful for mounting and interconnecting ceramic chip carriers.
2. Description of the Prior Art
Printed wiring boards are well known in the electronics industry including flexible printed wiring boards (U.S. Pat. No. 4,103,102) and rigid printed wiring boards. Essentially the substrate is a pressed laminate of fibrous reinforcing materials and thermosetting resins. The fibrous reinforcing materials normally are glass fibers, although other fibrous materials have been employed such as nylon, polyethylene terephthalate, aramid and the like. Thermosetting resins normally are polyepoxide resins selected for their thermal resistance although unsaturated polyester resins, polyimides and other thermoset resins have been employed. The printed wiring board is completed by application of electrical conductive material in appropriate patterns for receiving electrical components and electrical connectors. The use of electrical conductor patterns on both surfaces of the printed wiring board is commonplace.
The thermal expansion and contraction of the printed wiring board has been a matter of concern in order to achieve long-term reliability of the printed wiring board and its related components. In some installations it is important to maintain a relationship between the coefficient of thermal expansion of the printed wiring board and the coefficient of thermal expansion of the solder, plating and component leads which are mounted on the board (U.S. Pat. No. 3,700,538).
The coefficient of thermal expansion of the conventional glass fiber reinforced epoxy printed wiring boards is approximately 7.5 microinches/inch/.degree.F. This value is considered acceptable for attachment of axial leads, radial leaded, DIPS and flatpack components onto a printed wiring board.
A particular problem has occurred in attempts to secure leadless ceramic carriers for integrated circuit chips to printed wiring boards, called motherboards. Ceramic chip carriers have thermal expansion coefficients of 3.5 to 4 microinch/inch/.degree.F. To date, the use of ceramic chip carriers on glass-fiber reinforced wiring boards has been limited to small units where the success has been marginal because of the solder joint fatigue resulting from the differential thermal expansion coefficients between the ceramic chip carrier and the fiber reinforced plastic printed wiring board. The best success in using leadless ceramic chip carriers heretofore has been in installations where the ceramic chip carrier is secured to a ceramic printed wiring board. See "High-density chip carriers compete with DIPS," Electronic Products Magazine, November 1980, pgs. 27-32.
In order to use large ceramic chip carriers and to employ multiple ceramic chip carriers on a common printed wiring board, there is a need to produce a printed wiring board having an apparent coefficient of thermal expansion approximating that of the ceramic chip carrier yet retaining the coefficient of thermal expansion approximating that of the solder, plating and component leads in the thickness direction of the printed wiring board.